基本做法参照BITI的文章,http://blog.itpub.net/category/330/831

可结果发现并不对.过程如下:

SQL> set autotrace on
SQL> drop index t_index;

索引已删除。

SQL>  alter session set create_stored_outlines = my_demo;

会话已更改。

SQL> select count(object_id) from t;

COUNT(OBJECT_ID)
----------------
           49811

执行计划
----------------------------------------------------------
Plan hash value: 2966233522

---------------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |     1 |     5 |   159   (2)| 00:00:02 |
|   1 |  SORT AGGREGATE    |      |     1 |     5 |            |          |
|   2 |   TABLE ACCESS FULL| T    | 49811 |   243K|   159   (2)| 00:00:02 |
---------------------------------------------------------------------------

统计信息
----------------------------------------------------------
        175  recursive calls
          0  db block gets
        707  consistent gets
          0  physical reads
          0  redo size
        418  bytes sent via SQL*Net to client
        385  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          4  sorts (memory)
          0  sorts (disk)
          1  rows processed

SQL> alter session set  create_stored_outlines = false;

会话已更改。

SQL> create index t_index on t(object_id);

索引已创建。

SQL> select count(object_id) from t;

COUNT(OBJECT_ID)
----------------
           49811

执行计划
----------------------------------------------------------
Plan hash value: 789015712

--------------------------------------------------------------------------------
-

| Id  | Operation             | Name    | Rows  | Bytes | Cost (%CPU)| Time
|

--------------------------------------------------------------------------------
-

|   0 | SELECT STATEMENT      |         |     1 |     5 |    27   (4)| 00:00:01
|

|   1 |  SORT AGGREGATE       |         |     1 |     5 |            |
|

|   2 |   INDEX FAST FULL SCAN| T_INDEX | 49811 |   243K|    27   (4)| 00:00:01
|

--------------------------------------------------------------------------------
-

统计信息
----------------------------------------------------------
          1  recursive calls
          0  db block gets
        117  consistent gets
        110  physical reads
          0  redo size
        418  bytes sent via SQL*Net to client
        385  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed

SQL> alter session set use_stored_outlines = my_demo;

会话已更改。

SQL> select count(object_id) from t;

COUNT(OBJECT_ID)
----------------
           49811

执行计划
----------------------------------------------------------
Plan hash value: 789015712

--------------------------------------------------------------------------------
-

| Id  | Operation             | Name    | Rows  | Bytes | Cost (%CPU)| Time
|

--------------------------------------------------------------------------------
-

|   0 | SELECT STATEMENT      |         |     1 |     5 |    27   (4)| 00:00:01
|

|   1 |  SORT AGGREGATE       |         |     1 |     5 |            |
|

|   2 |   INDEX FAST FULL SCAN| T_INDEX | 49811 |   243K|    27   (4)| 00:00:01
|

--------------------------------------------------------------------------------
-

Note
-----
   - outline "SYS_OUTLINE_09061117273662511" used for this statement

统计信息
----------------------------------------------------------
          9  recursive calls
          3  db block gets
        118  consistent gets
          0  physical reads
        520  redo size
        418  bytes sent via SQL*Net to client
        385  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed

SQL>

说明:

我的数据库版本是10g,我发现10g在select count(*)时,不论是否建索引,也不论是否作统计数据收集,执行计划都是做全表扫描.所以只能换一种测试方法.存储做全表扫描的执行计划,在建立索引后,select count(索引列)也希望他走全表扫描来验证稳定执行计划.

但测试结果发现并非如预期一样.原因未知.